Response:

The source of the 2,300-year-old radiocarbon date (Keith and Anderson
1963, discussed by Strahler 1987, 156-157), has been abused and
misused to discredit radiocarbon dating.
The article discussed the potential errors that the presence of "dead
carbon" would introduce into the dating of mollusks. For example,
carbon dioxide in the water can partially come from Paleozoic
limestone, which lacks carbon-14. As a result, the carbon dioxide in
the water is deficient in carbon-14 relative to the atmosphere, and
mollusks living in the water build shells that give apparent dates
older than they really are. This is a type of "reservoir effect."

The 27,000 year old date comes from Riggs (1984, 224), who wrote:

Carbon-14 contents as low as 3.3 +/- 0.2 percent modern (apparent
age, 27,000 years) measured from the shells of snails Melanoides
tuberculatus living in artesian springs in southern Nevada are
attributed to fixation of dissolved HCO3- with
which the shells are in carbon isotope equilibrium.

In other words, the apparent age of 27,000 years for these snail shells
is another example of the reservoir effect. The springs, from which
the snails came, were fed by carbonate aquifers. As this water
percolated through the enclosing carbonates, it dissolved limestone and
dolomite hundreds of millions of years old. The dissolution of
limestone and dolomite introduced considerable quantities of "dead
carbon" into the groundwater. As a result, the groundwater which fed
the spring and in which the snails lived was significantly deficient in
carbon-14 relative to what is found in the atmosphere. When the snails
made their shells, they incorporated an excess amount of "dead carbon,"
relative to modern atmosphere, into their shells, which resulted in the
excessively old apparent date.

Contrary to the complaints of creationists, conventional scientists are
well aware of this problem. They test for it and take it into account
when interpreting radiocarbon data. In cases where corrections for
presence of dead carbon cannot be made, such dates are readily
recognized as erroneous and can be safely disregarded. This is not the
fatal flaw to radiometric dating that some creationists claim it to be.
It just shows that dates from mollusks from streams and lakes need to
be carefully evaluated as to their reliability. Other materials, such
as wood, charcoal, bone, and hide, would remain unaffected by this type
of reservoir effect. If found with shells in the same layer, these
materials could be dated to determine if shells are locally affected by
the reservoir effect and, if so, how much their radiocarbon dates have
been skewed by it.